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Endogenous retroviruses are one of the many threads that make up the tapestry of evolutionary evidence. They are "molecular remnants of a past parasitic viral infection".

The reason that they show evidence for evolution is that they can still be seen in the genomes of modern organisms, and they can still be recognised. Two species that share a particular retroviral pattern must be related to each other.

The kicker is that we share different retroviral sequences with different species. We share a particular sequence with new world monkeys. We share that sequence and a second sequence with old world monkeys. We share both of these sequences and a third sequence with gibbons. We share all three of these and a fourth with orangs. And so on. We share a fifth sequence with gorillas and a sixth with chimpanzees.

This is shown in this diagram, from the 29+ Evidences for Evolution article on TalkOrigins:

Endogenous retroviruses clearly demonstrate common ancestry, and thus evolution. A retrovirus infected the genome of the common ancestor of all the above primates before the lineages split from each other. Each lineage thus inherited this retroviral sequence. Then one of the subsequent lineages received another retroviral insertion, which all of its descendent lineages inherited. So the new world monkey lineage does not possess the inserted sequence that the old world monkeys, gibbons, gorillas, chimps and humans have. And so on. Common retroviral sequences can be seen in a wide range of different kinds of organism - not just the primates.

There is much more evidence for evolution than just this, though, and I would encourage anyone who doubts the existence of what we commonly refer to as a "mountain of evidence" to read the entire article 29+ Evidences for Macroevolution on the TalkOrigins website. Ashby Camp's response to this article can be found here, and the author's reponse to Camp's response can be found here.